Simulating The Precision Glass Molding Process

A temperature and strain rates dependent fracture model is developed based on Weibull statistics to quantitatively describe the brittle-ductile transition of glass fracture in precision glass molding process. Under the assistance of FEM simulation, this fracture model can be used to calculate the fracture probability of glass during the precision glass molding process. Meanwhile, the most probable fracture timing, location of fracture initiation and fracture pattern can be also predicted.

The study of manufacturing process of an infrared lens by using a commercial glass molding machine, GP-10000HT, is the second step to figure out all the details of a real precision glass molding process that help us to provide a simulation that makes a better prediction of glass molding process.

Molding tools in precision glass molding fail easily, even with protective thin film coatings applied. In this work, various efficient methods for assessing glass-coating interactions are developed, including a new, automated testing rig. Analysis of the testing results provides a better understanding of these mechanisms and how they are influenced by material properties and process parameters, so that the appropriate measures can be taken to prolong the life of the molding tools.

Precision glass molding is a net-shaping process to fabricate glass optics by replicating optical features from precision molds to glass at elevated temperature. The advantages of precision glass molding over traditional glass lens fabrication methods make it especially suitable for the production of optical components with complicated geometries, such as aspherical lenses, diffractive hybrid lenses, microlens arrays, etc. Despite of these advantages, a number of problems must be solved before this process can be used in industrial applications. The primary goal of this research is to determine the feasibility and performance of nonconventional optical components formed by precision glass molding. This research aimed to investigate glass molding by combing experiments and finite element method (FEM) based numerical simulations. The first step was to develop an integrated compensation solution for both surface deviation and refractive index drop of glass optics. An FEM simulation based on Tool-Narayanaswamy-Moynihan (TNM) model was applied to predict index drop of the molded optical glass. The predicted index value was then used to compensate for the optical design of the lens. Using commercially available general purpose software, ABAQUS, the entire process of glass molding was simulated to calculate the surface deviation from the adjusted lens geometry, which was applied to final mold shape modification. A case study on molding of an aspherical lens was conducted, demonstrating reductions in both geometry and wavefront error by more than 60%.

This book highlights the tools and processes used to produce high-quality glass molded optics using commercially available equipment. Combining scientific data with easy-to-understand explanations of specific molding issues and general industry information based on firsthand studies and experimentation, it provides useful formulas for readers involved in developing develop in-house molding capabilities, or those who supply molded glass optics. Many of the techniques described are based on insights gained from industry and research over the past 50 years, and can easily be applied by anyone familiar with glass molding or optics manufacturing. There is an abundance of information from around the globe, but knowledge comes from the application of information, and there is no knowledge without experience. This book provides readers with information, to allow them to gain knowledge and achieve success in their glass molding endeavors.

Molding process which would generate residual stresses within acceptable limits and design appropriate mold geometry that can yield the correct shape of the final glass optics.

Precision glass molding (PGM) is being considered as an alternative to traditional methods of manufacturing large-volume, high-quality and low-cost optical components. In this process, glass optics is fabricated by replicating optical features from precision molds to glass at elevated temperature. Chalcogenide glasses are emerging as alternative infrared materials for their wide range infrared transmission, high refractive index and low phonon energy. In addition, chalcogenide glasses can be readily molded into precision optics at elevated temperature, slightly above its glass transition temperature (Tg), which in general is much lower compared to oxide glasses. The primary goal of this research is to evaluate the thermoforming mechanism of chalcogenide glass around Tg and investigate its refractive index change and residual stresses in molded lens in and post PGM. Firstly, a constitutive model is introduced to precisely predict the material behavior in PGM by integrating subroutines into a commercial finite element method (FEM) software. This modeling approach utilizes the Williams-Landel-Ferry (WLF) equation and Tool-Narayanaswamy-Moynihan (TNM) model to describe (shear) stress relaxation and structural relaxation behaviors, respectively. It is predicted that `index drop’ occurred inside the molded prism due to rapid thermal cycling and the cooling rate above Tg can introduce large geometry deviations to the molded optical lens. Secondly, the refractive index variations inside molded lenses are further evaluated by measuring deviation angle through a prism & wavefront changes through molded lens using a Shack-Hartmann wavefront sensor (SHS), while the residual stresses trapped inside the molded lenses are obtained by using a birefringence method. Measured results of the molded infrared lenses combining numerical simulation provide an opportunity for optical manufacturers to achieve a better understanding of the mechanism and optical performance variation of chalcogenide glasses in and post PGM. Upon completion of the aforementioned research, two typical micro IR optics are designed, fabricated and tested, an infrared SHS and a large field-of-view (FOV) microlens array, as demonstrations. A novel fabrication method combining virtual spindle based high-speed single-point diamond milling and PGM process is adopted to fabricate infrared microlens array. The uniqueness of the virtual spindle based single-point diamond milling is that the surface features can be constructed sequentially by spacing the virtual spindle axis at an arbitrary position based on a combination of rotational and transitional motions of the machine tool. After the mold insert is machined, it is employed to replicate the optical profile onto chalcogenide glass. On the other hand, an infrared compound-eye system consisting of 3×3 channels for a FOV of 48°×48° is developed. The freeform microlens array on a flat surface is utilized to steer and focus the incident light from all three dimensions (3D) to a two-dimension (2D) infrared imager. Using raytracing, the profiles of the freeform microlenses of each channel are optimized to obtain the best imaging performance. To avoid crosstalk among adjacent channels, a micro aperture array fabricated by 3D printing is mounted between the microlens array and IR imager. The imaging tests of the infrared compound-eye imaging system show that the asymmetrical freeform lenslets are capable of steering and forming legible images within the design FOV. Compared to a conventional infrared camera, this novel microlens array can achieve a considerably larger FOV while maintaining low manufacturing cost without sacrificing image quality. Finally, two rapid heating processes are explored and demonstrated by using graphene-coated silicon as an effective and high-performance mold material for precision glass molding. One process is based on induction heating and the other one is based on mid-infrared radiation. Since the graphene coating is very thin (~45 nm), a high heating rate of 5~20 °C/s can be achieved. The contact surface of the Si mold and the polymer substrate can be heated above the Tg within 20 s and subsequently cooled down to room temperature within tens of seconds after molding. The feasibility of this process is validated by the fabrication of optical gratings, micropillar matrices, and microlens arrays on polymethylmethacrylate (PMMA) substrate with high precision. The uniformity and surface geometries of the replicated optical elements are evaluated using an optical profilometer. Compared with conventional bulk heating molding process, this novel rapid localized heating process could improve replication efficiency with better geometrical fidelity.

Abstract: Compression molding of glass is a promising manufacturing process for high precision, low cost glass optical elements. However, the conditions that glass molding processes are performed in are known to cause sticking between the glass work piece and the mold surface. When the glass-mold contact is repeated during heating-cooling cycles, some or all of a glass specimen often remains on a mold surface during de-molding and damages the formally high precision mold surface. In order to decrease the mold sticking and mold damage, mold materials for glass molds need to have high hardness, heat resistance, and chemical stability. Two examples of mold materials include tungsten carbide and silicon. The microstructures of popular mold materials such as WC-Co contain spots of a lower mechanical strength soft cobalt bonding agent that could adhere to the glass under high molding temperatures. To mitigate the problem, a thin layer coating of inert materials such as platinum or diamond like coating is deposited on the mold surface. In this research, two different coatings were applied to both tungsten carbide and silicon wafer substrates and then tested in a real molding environment. A new system was fabricated to test sticking force between the glass pieces and glass molds. Experiments demonstrated processing parameters including the level of compression, the time associated with compression and the time allowed for cooling significantly affected the glass to mold sticking force.